Highest Starting Current

Hello AntonA

This appears to be a Class or Homework Question, which should properly be in the http://cr4.globalspec.com/section/education subforum.

You have forgotten to state several other essential factors, which would enable any sort of proper reply.

Reply here, with

Kind Regards....

Hello Sparky

I don't see a problem (apart from it maybe being homework).

Sounds like a given motor wired 230 volt delta or 400 volt star, started DOL in either case.

So the FLC is √3 x higher in 230 volt delta, but starting current is same no. of times FLC in either case.

Cheers.......Codey

Hello Codemaster

Thank you for your reply above.

You have just thought about a DOL starter.

Perhaps the starter is a VFD, soft starter, resistance starter etc.

The Motor may be a Wound Rotor or other type of motor.

Without knowing what type starter and what type motor, we cannot assume a DOL starter plus a squirrel-cage rotor, as you have done.

Kind Regards....

Hello again Sparkstation

Possibly, but I'd have thought the ratio of starting to full-load current is about the same for star and delta connection, whatever the starter type (correct me if I'm wrong, I'm not a full-on elec. engineer!)

OP only asked which is higher starting current, but it seemed oversimplifying and perhaps misleading to just say "230 volt" without mentioning that 230 volt also has higher running current.

Regards to you too......Codey

Yes you are right, it is started directly.

That means that the starting current is also √3 times higher in 230 V delta?

Here's my problem:

A diesel generator (230 V) was installed for use in case of power shut down. One of the consumers is a motor for a fan. Then the system was changed to 400 V, so a transformer was installed. What happens now is that the generator can't deliver enough power to start this fan.

So when the staring current is lower for 400 V (star), then the problem is probably the transformer...

That means that the starting current is also √3 times higher in 230 V delta? Yes, but as the voltage is √3 times lower the power (and torque) are unchanged.

When the system was changed to 400 V, presumably the fan motor connection (and everything else on the system) was changed from delta to star? Seems unlikely it would be overlooked, and if it were there would be some signs in normal (mains) operation. But if that's OK, all I can think of is - is the generator sized very borderline, so transfomer losses are breaking the camel's back?

I assume the fan is the biggest motor, or you'd be having other problems. This should of course be started first to have best chance of getting it away.

Cheers.........Codey

starting current is a design function .Normally motors h have 6-7 times FLC+ tol as per BS ie 20%.Duration depends upon load.

I agree that a) this sounds like a homework question, and

b) There is not enough information to provide a meaningful answer.

"More" alone is not meaningful to start off with. More quantity of amps? More in percentage of amps? More in amps demand? What "more" is he referring to?

Do your own homework. Getting someone else to do it for you is a Management function, not an Engineering function.

So the starting current will be the same as long as there are 230 V over each stator coils (as it is in this case with 400 V (star) and 230 V delta)? And the operating current will be √3 times higher with 230 V (delta) compared with 400 V (star)?

JRaef: This is not a homework question... and I also thought this was a place where I could ask questions without being accused for asking too many questions...

So the starting current will be the same as long as there are 230 V over each stator coils (as it is in this case with 400 V (star) and 230 V delta)? Not sure what you mean by that. Each coil sees the same voltage in both types of connection (I think) but it doesn't follow that the current drawn from the supply is same for both. The line current (running and starting ) is √3 times higher with 230 V (delta), power and torque are same for both.

And the operating current will be √3 times higher with 230 V (delta) compared with 400 V (star)? Correct, as is the starting current.

Cheers......Codey

Thanks codemaster.

Then the starting current will be lower than with the previus 230 V installation. But the transformer have also the same relation, from 230 to 400, so the current should be √3 times higher on the primary side.

This should give the same current as previously, except maybe for some % loss from the transformer...

Sorry, it did sound like a homework question and we get a lot of that here. You also did not explain, even yet, why you want to know or the context of your question. Yes, this is a good place to ask questions, but it is also a good idea to be forthcoming with information if you want a good answer. We don't charge you by the word after all...

Current, as a PERCENTAGE of motor FLA will be exactly the same regardless of voltage.

The AMOUNT of amps will be higher by the √3 at 230V, both for starting AND for running.

I'm still curious as to the reason you are asking. Usually, your supply is your supply, you don't typically get a chance to select which one you want, so the question would be moot. But if you are in a position to choose, go with the higher voltage. The control equipment will be smaller and less expensive and the I²R operating losses will be lower. Starting current however is the LEAST of the issues involved in that choice however.

The problem comes from a production plant where earlier all the equipment used 230 V (there was no 400 V in that part of the plant). In case of power failure a diesel generator would start and this one also delivered 230 V. Among the equipment that should start in case of power failure was a quite heavy fan.

Then the supply was changed to 400 V for this part of the plant so a transformer was installed for transforming the 230 V from the diesel generator up to 400 V for the equipment.

After this change the problems with starting the fan occure. The motor and the load is the same as earlier but the diesel generator stops because of overload. The diesel generator stops because of low voltage (it cannot produce 230 V with 50 HZ).

If the motor for this fan is disconnected then everything works fine and the rest of the equipment connected to the diesel generator starts.

Hello AntonA

Thank you for explaining further re the installation and problem.

In trying to solve a problem via "Remote Control", it is essential that full details are given, so that the best possible answers are given.

At the outset, and even with your further two posts, you have omitted to advise those important details.

That is why I reply as below, and trust you are not offended by my asking for the details.

Summary of your situation: It appears that the 230V Alternator is unable to correctly start/run the fan, with your present system.

A. Possible Remedies:

1. Obtain a larger generator set, (genset) with 400 V 3 phase output, connect the fan to it: No intervening 230V/400V transformer = No worries (Except the cost and inconvenience)
   
2. Install a smaller fan: May not be possible for reasons of required ventilation of the works
   
3. Use a VFD or Soft Starter - Should work OK - would need to be carefully checked and settings optimised

B. Fan Details needed:You have not advised generating set output, nor Fan Motor/type size, (except a "heavy fan").

1. What mass (weight) is the fan assembly?
2. What speed does it rotate at?
3. What diameter is the fan assembly?
4. Advice of the above would enable better understanding of the situation.

It would be appreciated if you could supply the following:

C. Genset output at 50Hz in kW

• Nameplate details
  

D. Motor nameplate details:

1. kW
2. Speed
3. Type:

• Squirrel cage
• Slipring
• Other - Advise please.
  

It may be possible that the 230V generator alternator has sufficient output to run a fan motor of slightly less kW than the kW output of the genset.

It is best never to exceed a genset load of greater than 80% of full load genset capacity, as System instability can arise, particularly with inductive loads, and automatic tripping/shutdown of the genset.

The act of interposing a 230V/400V transformer between the genset and the fan will reduce the useful output of the genset to approximately 60% - 70% of the rated genset output during a large (we don't yet know how 'large') fan motor start, because of the extra work for the genset of magnetising/demagnetising that transformer core iron.

That scenario may mean the genset trips out, merely because of the extra iron losses in the circuit, caused by the intervening 230V/400V transformer.

Note: The rating of that intervening 230V/400V transformer would need to be some 15% - 20% larger than the motor rating, in kW, to allow for transformer losses, dependent on the actual kW rating of the fan motor and size of fan attached (Details yet to be provided - see above please).

Reply here, with

Kind Regards....

AntonA;

Here I want to ask another one question that the all the essential loads are connected to 230/400 V transformer or only the fan which causing the problem. And what the total load of all essentials?

You are telling it is heavy motor, then a transformer connected with a heavy motor definitely will draw heavy inrush which will not met by your DG.

I think your plant also have redundant power supply to take care of the important loads. And when you are saying DG set, then there is total power failure, and what ever may be the important operation, it cause the tripping of the fan.However continuous running was not ascertained, I think there is no such a big problem if it is not started with full load. So it is better to start the motor with any type of starter ( that depends..) except DOL starter. I think with this your DG can run your motor wiht out any problem.

Here's some data I have, both from tests and nameplates:

When the generator starts after a power failure then this will happen:

1. Lighting (25 kW)

2. A smaller fan will start, and after stabilizing the total power consumption is 40 kW

3. Then the larger fan will start. Nominal current for this motor is 118 A with 230 V.

Measurements done on this motor:

Normal run: 55 A (with 400 V)

Startup: 430 A in 8 seconds

By reading on the instrument panel of the generator when only the large fan is started: 700-800 A and 190 kW for some seconds, before the generator stops bacause of "Low generator voltage", as is the message on the panel.

Generator data:

254/279 kVA

cos(phi) = 0.8

P= 221 KW

1500 r/min

Transformator data:

230/400V 315 kVA

Hello AntonA,

Are you got your problem solved. If not try this one:

1. After starting of DG set, Charge your transformer with no load and connect the heavy load first. By this high starting current of the load can be withstood by Generator by its reserve capacity.
2. Then connect the loads one by one starting from higher one to lower.

I am just conveying this one as I am not aware of your current procedure. However if the DG got trip even when after step 1, there is no option else you have change your network elements.

And you are having a opportunity to solve this problem by any means, and ofcourse that will serve as a reference in future not only for you.

Favouring a Replay.

See how much better it is when we can see more of the problem?

Quote from Sparkstation
"The act of interposing a 230V/400V transformer between the genset and the fan will reduce the useful output of the genset to approximately 60% - 70% of the rated genset output during a large (we don't yet know how 'large') fan motor start, because of the extra work for the genset of magnetising/demagnetising that transformer core iron."

Since your fan started before and does not any longer, I think this issue is the crux of the matter (although I think that 60-70% is a bit extreme). Whenstill at 230V your emergency power system was apparently marginally capable of starting that motor DOL, but with the added burden of the transformer it is no longer able to. My suggestions, most of which echo other people's comments, are:

1. Start by shedding other loads when the fan has to start. In other words, install some inexpensive automation such as a "smart relay" that detects when the system is on Generator power so that it can shut down all non-essential loads until after that fan has started. That may not help if the fan has to cycle on and off while on Generator power though.
2. Buy a soft starter for that fan. Since you were marginally able to start it without the transformer losses before, a slight lowering of the starting burden by using a soft starter will undoubtedly be all you need.
3. Evaluate your process needs and decide if you can afford a VFD. Fans are often prime candidates for the excellent energy savings capabilities of VFDs and in many cases, a VFD will pay for itself in under 2 years just on the energy savings alone. Then once you have justified it on that basis, the VFD will be even better than a soft starter at preventing the Generator overload. In other words, you need to spen SOME money to take care of this, so the wisest investment is the one that saves in some other way.

Good luck.

Hello JRaef

The 60% to 70% is total system efficiency, with a smaller genset alternator/transformer/fan motor.

As the size of all above increases, efficiency also increases.

Because we have yet to be advised of definite details, I have allowed for the worst in % of total efficiency.

Kind Regards....

Yes, after I posted that I thought about it some more and realized you could be thinking of the worst case scenario.

JRaef..

I believe option #3 is the best.....

If this "Large Fan" uses damper or something similar to control the flow of air. There is a good amount of $$$$ to be saved. It's just a matter of a company making the initial investment to pay themselves back for years to come. I have seen initial investment pay off as little as 8 months. That was 400HP (300KW) fan saving a little more than 50% of energy consumption. Also "start up" current is minimal unless the process requires short accel times but even with that, "starting" current is dramatically lower.

All very fantastic explanations, gentlemen. For me the answer is quite simple. The starting current of a motor is 7 times the nominal current. (7 times the Amp rating wich is on the identification plate of the motor) Motor protectors are build to stand 7 times the the current of its setting during startup.

No matter if you have your motor running in star or delta, 7 is the factor. Of course this numner is going to be √³ smaller if the same motor is connected in star than in delta ... but 7 IS ALWAYS THE FACTOR for any asynchronous 3 phase motor.

Hello 44mEurope

You are generalising with the figures quoted in your above reply.

You assume that the motor is a squirrel cage rotor, and a DOL Starter, which gives the approximations you quote above.

While generalisations may be assumed, it is good engineering practice to deal in definite details.

Evidently you have not met VFD or Soft Starters, or other current minimising Starters.

Kind Regards....

yes of course Sparkstation man ... I generalised the answer. I do know there's lotsa possabilities to reduce starting currents of motors, and of course there are several types of motors, all having their specific parameters.

But if the original question is a "general" question, I come up with a "general" answer, yes ? I think that you'll agree with me that when you have "a switch" and a motor, seven is the magic number, right ? I seem to remember that there is a calculation that leads to this number, but sorry guys, I only remember the result.

I think that for special/specific motors or applications it's always good to consult the manufactorer of the motor. Only there you can find correct and specific answers to specific questions about that motor, yes ?

Anyhow ... I wanna thank the gentlemen who gave those fantastic explications. I bookmarked this thread because there is A LOT of very useful information to read about motors here.

^5 Sparkstation !

Dear participants,

I think the delta ot star connections don't change the starter current because the voltage changes accordingly.

The problem is, from my point of view, the transformer rating. We didn't listen here its rating numbers(voltage, VA) compared with fun rating.

The second - the usage of smart starter will not help. Because the fun starting is the easiest type of starting current problem. The resistive moment of fun depends on its rotating velosity very strong. Multipled by square of the velosity. So, starter current will not rise up 7 times as in the other cases. It will be much less.

Is there a prize for the right answer?